Evidence for gravitational lensing of GRB 200716C

TL;DR

This paper presents evidence that GRB 200716C's two pulses are likely caused by gravitational lensing, with Bayesian analysis estimating the lensing parameters and suggesting a possible lensing object of about 2.4×10^5 solar masses.

Contribution

The study provides the first Bayesian analysis indicating gravitational lensing in a GRB with detailed temporal and spectral evidence, estimating lens parameters and false alarm probability.

Findings

Bayesian analysis supports gravitational lensing hypothesis.

Estimated lens mass is approximately 2.4×10^5 solar masses.

False alarm probability for lensing detection is about 0.07%.

Abstract

Observationally, there is a small fraction of Gamma-ray bursts (GRBs) with prompt emission observed by Fermi/GBM that are composed of two pulses. Occasionally, the distance to a GRB may be lensed when a high mass astrophysical object resides in the path between the GRB source and observer. In this paper, we describe GRB 200716C, which has a two-pulse emission and duration of a few seconds. We present a Bayesian analysis identifying gravitational lensing in both temporal and spectral properties, and calculate the time delay ($\Delta t\sim 1.92$ s) and magnification ($\gamma\sim 1.5$) between those two pulses based on the temporal fits. One can roughly estimate the lens mass to be about $2.4\times 10^{5}~M_{\odot}$ in the rest frame. We also calculate the false alarm probability for this detection to be about 0.07\% with trial factors, and a present-day number density of about $808 \rm~Mpc^{-3}$ with an energy density $\Omega\sim 1.4\times 10^{-3}$. If the first pulse of this GRB near the trigger time is indeed gravitationally echoed by a second pulse, GRB 200716C may be a short GRB candidate with extended emission.